Depends on what you want from an estate and what sort of level of running costs you're prepared to incur....
If you want space, I don't think you can beat a Mondeo estate. a lot of car for the money, low running costs, HUGE amount of load space (more than an E39 estate) - this thing can swallow chests, wardrobes and other weird things that no other estate can. Only downside is the poor range of engines, particularly with the TDCi reliability issues at this price point.
E39 Estate - great car but not a great deal of load space which is narrow, high running costs at this price point, and you're not going to get the best example for £4k. Same applies with the E46 Estate with the only difference being the smaller load space.
Volvo V70 - do you really want to drive a bus?
Personally, I don't think you can beat an A4 (B6) Avant as the best all round 'prestige' proposition (provided maximum space isn't your main criterion) but you really will struggle to find a good B6 Avant for £4k - they're all going to be high mileage examples. The TDi 130 also smokes like a chimney at average to high mileages.
As for another car that might be worth considering from a 'non-prestige' manufacturer, the Honda Accord Estate with the i-CDTi engine could be a good option. Honda build quality and reliability with a decent diesel engine. Electric rear tailgate and a 'single press' lever to fold down both rear seats inc seat squabs. Only downside is that it looks a bit ugly in estate guise and load space is nowhere near as much as the Mondeo. A Mazda 6 Estate might be a cheaper alternative. The 'banger' alternative would the old C5 Estate, which is not only cheap to buy but offers a decent load area and better ride quality compared to the Audi.
Leftfield alternatives would be the 407 Estate (you can pick up late examples at auction for not very much money) or the Subara Legacy Estate. Don't have any experience of how either one of them drive or load up though.